Polychloroprene-based adhesives have been mostly solvent-type adhesives, which are mainly used as dissolved in an organic solvent such as toluene, hexane, ethyl acetate, methylethylketone, cyclohexane or the like, and thus, there was a concern about environmental pollution and the like. Thus, so-called aqueous adhesives, which do not use any organic solvent, have been developed eagerly, but adhesives made with traditional polychloroprene latexes had a problem that the adhesiveness and the water resistance thereof were unsatisfactory.
Traditional polychloroprene latexes were produced via the so-called emulsion polymerization mechanism, as chloroprene or a mixture of chloroprene and a monomer copolymerizable with chloroprene is emulsified in water with an emulsifier such as rosin acid soap, sodium alkylsulfate, higher alcohol sulfate ester sodium salt, polyoxyethylene alkyl ether, alkyl amine salt, quaternary ammonium salt or polyvinylalcohol and polymerized by addition of a radical initiator such as potassium persulfate. However, the mechanism generally demands a great amount of the emulsifier and thus, there was pointed out a problem of significant deterioration in adhesion properties and water resistance due to inhibition of the adhesiveness inherent to the polychloroprene. For that reason, there have been many attempts to develop a method of producing a polychloroprene latex having a reduced emulsifier content.
A known method of obtaining polymer particles without use of an emulsifier is soap-free polymerization. The soap-free polymerization is a method of producing polymer fine particles in water as solvent without addition of a soap and has been used as a technology favorably applicable for production of pollution-free adhesives and bonding agents and inks. In short, the difference between the emulsion polymerization and the soap-free polymerization is the presence or absence of the micelles of the soap molecule, and polymerization with an emulsifier in an amount of less than its critical micelle concentration (CMC) is also a soap-free polymerization, when seen from the viewpoint of polymerization mechanism. The boundary between them can be identified easily by the change in the number of the polymer particles generated when polymerization is carried out at different emulsifier concentrations, and the soap-free polymerization can give more large-sized and monodispersed particles than the emulsion polymerization.
The surfactant is dispersed ionically in an aqueous solution when the concentration is quite low, but forms stable micelles by association in the aqueous solution when the concentration increases and reaches the saturation point of dispersion. The micelle-forming concentration, which resides in a very narrow range, is called critical micelle concentration (hereinafter, referred to simply as CMC). The CMC varies according to the kind of the surfactant, the temperature of the aqueous surfactant solution, the salt concentration in the aqueous surfactant solution and other external conditions, but is normally about 10−5 mol/l to 10−2 mol/l. CMC's of various surfactants are described in detail in Surfactant Handbook, Revised Edition (published by Kougakutosho Ltd.) However, there has been no disclosure on soap-free polymerization of chloroprene.
For example, a method of polymerizing a hydrophobic vinyl monomer in an aqueous medium in the absence of emulsifier is disclosed as a method similar to the soap-free polymerization (Patent Document 1). However, disclosed therein is mostly description on monomers such as styrene, methyl methacrylate and n-butyl methacrylate and there is no description at all about soap-free polymerization of chloroprene, and in addition, it does not describe the inherent problem that soap-free polymerization of chloroprene in the absence of emulsifier results in formation of a great amount of deposits.
For example, a method of using a polychloroprene-based random copolymer having a hydrophilic group additionally (Patent Document 2) and a method of using additionally a polychloroprene-based polymer having a acidic functional group at the terminal in the presence of a suitable amount of a hydrophilic solvent (Patent Document 3) are reported as the processes of producing a polychloroprene latex containing almost no emulsifier (soap-less polychloroprene latex).
However, all of these methods use a chloroprene-based polymer having a surface activity for compensation of insufficient emulsifying force, and polychloroprene latexes are produced by emulsion polymerization, when seen from the viewpoint of polymerization mechanism. The method of the present invention, in which the chloroprene latex is produced via the soap-free polymerization mechanism, is significantly different in polymerization mechanism from the technologies above. In addition, these technologies are two-step production methods having a step of preparing a terminally functionalized polychloroprene-based copolymer and a step of preparing a polychloroprene latex by using the polychloroprene-based copolymer obtained and thus have an industrially disadvantageous aspect.